Optical fibers made with synthetic fused silica have found great many uses in military, medical, industrial and other applications primarily because fused silica has a wide transmission range from 190 nm in the ultraviolet to 2500 nm in the infrared region of the electromagnetic spectrum. It is known that various defects in the structure of the glass form both during manufacture of the bulk glass and during the fiber drawing process. Other defects result from impurities present in the glass. These defects remain in the final fiber product and cause absorption of some light in numerous wavelengths. Ultraviolet radiation, in wavelengths lower than 300 nm in particular, transmitting through the fiber is able to generate defects, commonly called e prime centers which gradually degrade transmission of the glass to such an extent that fiber becomes useless in the 190-250 nm range in a matter of days. It has been known for some time now that some of these defects can be healed by soaking fiber in a hydrogen atmosphere for 10-20 hours. Additionally, even draw induced losses appearing as an absorption band at about 630 nm in dry silica fiber, that is silica fiber with very low OH content, can be made to disappear with this process. This particular trap (defect) is of a negative charge (extra electron) and is neutralized by a proton (H+). These traps are formed in the silica matrix during fiber draw and are filled or neutralized by hydrogen diffusing into them.
The effect of UW radiation induced absorption or coloration is sometimes referred as "formation of color centers" in silica. It has been shown that if fiber or silica that has been damaged in this way is heated to a certain temperature, then the damage can be annealed out. Strong gamma rays have also been shown to eliminate this effect.
The diffusion of hydrogen into fused silica has been studied extensively. The diffusion coefficient increases with higher temperatures, as expected. In communication fibers, the presence of hydrogen is not desirable, since it causes absorption bands at communication wavelengths, degrading system transmission. Also, fibers treated in this way develop surface microcracks that can grow by a hydrolysis mechanism when fibers are under stress. See, for example, "Diffusion of Hydrogen and Deuterium in Fused Quartz", R. W. Lee, R. C. Frank, and D. E. Wests, The Journal of Chemical Physics, Vol. 36, No. 4, pgs. 1062-1071, Feb. 15, 1962, and "Reliability of Optical Fibers Exposed to Hydrogen: Prediction of Long-term Loss Increases", Paul J. Lemaire, Optical Engineering, Vol. 30, No. 6, pgs. 780-788, June 1991, incorporated herein by reference.
According to conventional thinking, therefore, it is imperative to keep hydrogen and OH (hydroxyl ion) out of communication fiber. Hermetic coatings have been developed that slow the diffusion process by orders of magnitude, in an attempt to extend the service life of the fibers.
An excess of hydrogen in the glass matrix has been known, in situ, to immediately heal UV radiation induced defects. Indeed, current methods to manufacture solarization resistant fiber involve soaking the fiber in hydrogen at some elevated temperature to create this hydrogen rich environment. However, the energy available at room temperature causes excess hydrogen to diffuse out of the fiber gradually. This causes the fiber to lose its solarization resistance in several months.